1. Field of the Invention
The present invention is directed generally to semiconductor devices and, more particularly, to memory devices which include high resolution trimable output drivers.
2. State of the Art
Semiconductor devices such as microcomputers, memories, gate arrays, among others, include input/output pins and an output circuit for transmitting data to other devices along transmission lines formed on a board, and the like. A circuit within the semiconductor device that is responsible for transmitting data includes, for example, output buffers and drivers. For optimum transmission, the impedance of the transmitting device should be matched to the impedance of the transmission network and receiving device.
As operational speeds of electronic devices increase, the swing of transmitted signals decreases. However, as the signal swing width of a transmitted signal decreases, external noise increases. External noise can affect the reflection characteristics of an output signal if there is any impedance mismatch at an interface. Impedance mismatches may be caused by external noise, noise on a supply voltage, temperature and process variations, as well as other variations. If an impedance mismatch arises, the transmission speed of the data decreases, and the data from a semiconductor device may become distorted. Thus, in a case where a semiconductor device receives distorted data, problems can be caused by setup/hold failures or errors in reading received data.
Integrated circuits typically include a number of input/output terminals or pins which are used for communication with additional circuitry. For example, an integrated memory device such as a dynamic random access memory (DRAM) includes both control inputs for receiving memory operation control signals, and data pins for bidirectional data communication with an external system or processor. The data transmission rate of conventional integrated circuits is primarily limited by internal circuitry operating speeds. That is, communication networks have been developed which can transmit signals between circuitry at a rate that is faster than the capacity of many integrated circuits.
To address the need for faster circuits, a group of integrated circuits can be combined on a common bus. In this configuration, each integrated circuit operates in a coordinated manner with the other integrated circuits to share data which is transmitted at a high speed. For example, a group of memory devices, such as DRAMs, static RAMs, or read only memories (ROMs), can be connected to a common data bus. The data rate of the bus may be substantially faster than the feasible operating speed of the individual memories. Each memory, therefore, is operated so that while one memory is processing received data, another memory is receiving new data. By providing an appropriate number of memory devices and an efficient control system, very high speed data transmissions can be achieved.
In order to reduce the effects of impedance mismatches, techniques for more tightly matching the output driver impedance with the characteristic impedance of the remaining circuit within which the output driver interacts are desirable. Manufacturing process control during fabrication of the integrated circuit that includes an output driver is one method for controlling the output impedance of the output driver. However, as transmission data rates increase, impedance matching of the output driver to the characteristic impedance using conventional processing controls is inadequate.
For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for a high speed output driver circuit wherein the impedance may be more precisely adjusted.